The present invention relates generally to improved drill string drill collars and their manufacture, and more specifically relates to drill string survey collars, each formed of a plurality of materials to facilitate improved resistance to stress corrosion cracking and corrosion.
When magnetic survey instruments are utilized to survey a well bore, such as with measurement-while-drilling operations, non-magnetic drill collars, also known as "survey collars," must be utilized in the drill string to isolate the surveying instruments from magnetic disturbances which would otherwise be created by the drill pipe and other magnetic components in the well. Generally, the practice has been to make the survey collars from austenitic stainless steel.
In the mid- to late-1970s, drilling companies began to use high chloride muds in their drilling opertions. While this use has had an advantageous effect on drilling operations, it has had the detrimental side effect of inducing corrosion in the survey collars and other equipment used in the hole. In addition, it was discovered that it contributed to the stress corrosion cracking of the survey collars, thus reducing their life from a preferable 6-8 years to as little as 6-8 months.
Various methods have been tried by the industry to alleviate both the corrosion and the stress corrosion cracking problems. One early solution has been to make the non-magnetic survey collars from various special alloys which exhibit improved resistance to corrosion and/or to stress corrosion cracking. These alloys, while accomplishing in some part both of these objectives, suffered from a number of other problems. For example, the characteristics of the alloys required to make them both non-corrosive and resistant to stress corrosion cracking tended also to make the collars more susceptible to "galling" of the threads, which damaged the surfaces of the threads. This susceptibility to galling meant the collars required frequent remachining which resulted in down time and repair costs that were unacceptable. In addition, drill collars made from these alloys were less successful at alleviating the stress corrosion cracking problems than they were in alleviating the corrosion problem.
Another attempt to solve the problem was to return to using collars of austenitic stainless steel, which were less subject to galling, and to induce both axial compressive stress and hoop compressive stress in them. This axial and compressive hoop stress has been performed by "peening" the interior of the survey collar with peening shot or a peening hammer or by rolling tools. This peening effectively deforms the interior surface of the collar axially, circumferentially, and radially to effectively apply both compressive stress and hoop stress to the interior of the collar. The application of these stresses substantially alleviated the stress-cracking problems. However, this partial solution did not alleviate the corrosion problem.
Accordingly, the present invention provides a new method and apparatus which provides non-magnetic survey collars, and methods for their manufacture, which exhibit improved resistance to stress corrosion cracking and corrosion and yet which maintain optimal structural strength.